The goals of this Program are to broaden the application and increase success and safety of allogeneic hematopoietic cell transplantation (HCT) after nonmyeloablative conditioning in treating of patients with hematologic malignancies. To this end, we propose two preclinical and two clinical projects. The preclinical Projects 1 and 2 involve a canine model of HCT with a long history of clinical translation. Project 1, which developed the clinical HCT regimen used in Projects 3 and 4, will address three major issues in allogeneic HCT. One is to replace the cytotoxic conditioning regimen with biological means of tolerance induction to donor grafts and thereby reduce late regimen-related sequela. Another is to explore novel ways of preventing graft-vs.-host disease (GVHD) that will avoid the need for and side effects from current long-term post-grafting immunosuppression. The third is to improve eradication of persistent malignancies as seen in patients transplanted under Projects 3 and 4. This third aim will use mixed donor/host hematopoietic chimerism and experimentally-induced leukemia as models of persisting malignant cells and, in collaboration with Project 2, investigate how to enhance graft-vs.-tumor effects without risking GVHD. Project 2 will use genomics approaches to identify canine minor histocompatibility antigens with the goal of discriminating between those antigens whose expression is restricted to hematopoietic cells and those which are ubiquitously expressed. Knowledge generated in this project will increase our understanding of GVHD and graft-vs.-tumor effects. Projects 3 and 4 use allogeneic HCT to treat human patients with advanced hematologic malignancies. The HCT regimen uses truly nonmyeloablative conditioning as evidenced by autologous marrow recovery in those rare patients who reject their grafts. It has minimal early toxicities and, importantly, allows for the purest determination of graft-vs.-tumor effects apart from conditioning and the best determination of GVHD not augmented by regimen-related toxicities. It provides an excellent foundation on which to add disease and disease stage specific modalities, which will include immune manipulations in Project 3 and pharmacological manipulations in Project 4.The public health benefits of the Program are underscored by the fact that, since the clinical introduction of the nonmyeloablative regimen, more than 1,200 patients with various malignant and nonmalignant blood disorders have benefited from treatment by allogeneic HCT who otherwise would have been excluded because of age and co-morbidities. This is especially important since median ages at diagnosis of patients with most candidate diseases range from 65 to 70 years, which is beyond the age range of inclusion in conventional myeloablative HCT regimens.